Tag Archives: sha1

MD5 and SHA-1 Still Used in 2018

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2018/12/md5_and_sha-1_s.html

Last week, the Scientific Working Group on Digital Evidence published a draft document — “SWGDE Position on the Use of MD5 and SHA1 Hash Algorithms in Digital and Multimedia Forensics” — where it accepts the use of MD5 and SHA-1 in digital forensics applications:

While SWGDE promotes the adoption of SHA2 and SHA3 by vendors and practitioners, the MD5 and SHA1 algorithms remain acceptable for integrity verification and file identification applications in digital forensics. Because of known limitations of the MD5 and SHA1 algorithms, only SHA2 and SHA3 are appropriate for digital signatures and other security applications.

This is technically correct: the current state of cryptanalysis against MD5 and SHA-1 allows for collisions, but not for pre-images. Still, it’s really bad form to accept these algorithms for any purpose. I’m sure the group is dealing with legacy applications, but I would like it to really push those application vendors to update their hash functions.

Engineering deep dive: Encoding of SCTs in certificates

Post Syndicated from Let's Encrypt - Free SSL/TLS Certificates original https://letsencrypt.org/2018/04/04/sct-encoding.html

<p>Let&rsquo;s Encrypt recently <a href="https://community.letsencrypt.org/t/signed-certificate-timestamps-embedded-in-certificates/57187">launched SCT embedding in
certificates</a>.
This feature allows browsers to check that a certificate was submitted to a
<a href="https://en.wikipedia.org/wiki/Certificate_Transparency">Certificate Transparency</a>
log. As part of the launch, we did a thorough review
that the encoding of Signed Certificate Timestamps (SCTs) in our certificates
matches the relevant specifications. In this post, I&rsquo;ll dive into the details.
You&rsquo;ll learn more about X.509, ASN.1, DER, and TLS encoding, with references to
the relevant RFCs.</p>

<p>Certificate Transparency offers three ways to deliver SCTs to a browser: In a
TLS extension, in stapled OCSP, or embedded in a certificate. We chose to
implement the embedding method because it would just work for Let&rsquo;s Encrypt
subscribers without additional work. In the SCT embedding method, we submit
a &ldquo;precertificate&rdquo; with a <a href="#poison">poison extension</a> to a set of
CT logs, and get back SCTs. We then issue a real certificate based on the
precertificate, with two changes: The poison extension is removed, and the SCTs
obtained earlier are added in another extension.</p>

<p>Given a certificate, let&rsquo;s first look for the SCT list extension. According to CT (<a href="https://tools.ietf.org/html/rfc6962#section-3.3">RFC 6962
section 3.3</a>),
the extension OID for a list of SCTs is <code>1.3.6.1.4.1.11129.2.4.2</code>. An <a href="http://www.hl7.org/Oid/information.cfm">OID (object
ID)</a> is a series of integers, hierarchically
assigned and globally unique. They are used extensively in X.509, for instance
to uniquely identify extensions.</p>

<p>We can <a href="https://acme-v01.api.letsencrypt.org/acme/cert/031f2484307c9bc511b3123cb236a480d451">download an example certificate</a>,
and view it using OpenSSL (if your OpenSSL is old, it may not display the
detailed information):</p>

<pre><code>$ openssl x509 -noout -text -inform der -in Downloads/031f2484307c9bc511b3123cb236a480d451

CT Precertificate SCTs:
Signed Certificate Timestamp:
Version : v1(0)
Log ID : DB:74:AF:EE:CB:29:EC:B1:FE:CA:3E:71:6D:2C:E5:B9:
AA:BB:36:F7:84:71:83:C7:5D:9D:4F:37:B6:1F:BF:64
Timestamp : Mar 29 18:45:07.993 2018 GMT
Extensions: none
Signature : ecdsa-with-SHA256
30:44:02:20:7E:1F:CD:1E:9A:2B:D2:A5:0A:0C:81:E7:
13:03:3A:07:62:34:0D:A8:F9:1E:F2:7A:48:B3:81:76:
40:15:9C:D3:02:20:65:9F:E9:F1:D8:80:E2:E8:F6:B3:
25:BE:9F:18:95:6D:17:C6:CA:8A:6F:2B:12:CB:0F:55:
FB:70:F7:59:A4:19
Signed Certificate Timestamp:
Version : v1(0)
Log ID : 29:3C:51:96:54:C8:39:65:BA:AA:50:FC:58:07:D4:B7:
6F:BF:58:7A:29:72:DC:A4:C3:0C:F4:E5:45:47:F4:78
Timestamp : Mar 29 18:45:08.010 2018 GMT
Extensions: none
Signature : ecdsa-with-SHA256
30:46:02:21:00:AB:72:F1:E4:D6:22:3E:F8:7F:C6:84:
91:C2:08:D2:9D:4D:57:EB:F4:75:88:BB:75:44:D3:2F:
95:37:E2:CE:C1:02:21:00:8A:FF:C4:0C:C6:C4:E3:B2:
45:78:DA:DE:4F:81:5E:CB:CE:2D:57:A5:79:34:21:19:
A1:E6:5B:C7:E5:E6:9C:E2
</code></pre>

<p>Now let&rsquo;s go a little deeper. How is that extension represented in
the certificate? Certificates are expressed in
<a href="https://en.wikipedia.org/wiki/Abstract_Syntax_Notation_One">ASN.1</a>,
which generally refers to both a language for expressing data structures
and a set of formats for encoding them. The most common format,
<a href="https://en.wikipedia.org/wiki/X.690#DER_encoding">DER</a>,
is a tag-length-value format. That is, to encode an object, first you write
down a tag representing its type (usually one byte), then you write
down a number expressing how long the object is, then you write down
the object contents. This is recursive: An object can contain multiple
objects within it, each of which has its own tag, length, and value.</p>

<p>One of the cool things about DER and other tag-length-value formats is that you
can decode them to some degree without knowing what they mean. For instance, I
can tell you that 0x30 means the data type &ldquo;SEQUENCE&rdquo; (a struct, in ASN.1
terms), and 0x02 means &ldquo;INTEGER&rdquo;, then give you this hex byte sequence to
decode:</p>

<pre><code>30 06 02 01 03 02 01 0A
</code></pre>

<p>You could tell me right away that decodes to:</p>

<pre><code>SEQUENCE
INTEGER 3
INTEGER 10
</code></pre>

<p>Try it yourself with this great <a href="https://lapo.it/asn1js/#300602010302010A">JavaScript ASN.1
decoder</a>. However, you wouldn&rsquo;t know
what those integers represent without the corresponding ASN.1 schema (or
&ldquo;module&rdquo;). For instance, if you knew that this was a piece of DogData, and the
schema was:</p>

<pre><code>DogData ::= SEQUENCE {
legs INTEGER,
cutenessLevel INTEGER
}
</code></pre>

<p>You&rsquo;d know this referred to a three-legged dog with a cuteness level of 10.</p>

<p>We can take some of this knowledge and apply it to our certificates. As a first
step, convert the above certificate to hex with
<code>xxd -ps &lt; Downloads/031f2484307c9bc511b3123cb236a480d451</code>. You can then copy
and paste the result into
<a href="https://lapo.it/asn1js">lapo.it/asn1js</a> (or use <a href="https://lapo.it/asn1js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this handy link</a>). You can also run <code>openssl asn1parse -i -inform der -in Downloads/031f2484307c9bc511b3123cb236a480d451</code> to use OpenSSL&rsquo;s parser, which is less easy to use in some ways, but easier to copy and paste.</p>

<p>In the decoded data, we can find the OID <code>1.3.6.1.4.1.11129.2.4.2</code>, indicating
the SCT list extension. Per <a href="https://tools.ietf.org/html/rfc5280#page-17">RFC 5280, section
4.1</a>, an extension is defined:</p>

<pre><code>Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
— contains the DER encoding of an ASN.1 value
— corresponding to the extension type identified
— by extnID
}
</code></pre>

<p>We&rsquo;ve found the <code>extnID</code>. The &ldquo;critical&rdquo; field is omitted because it has the
default value (false). Next up is the <code>extnValue</code>. This has the type
<code>OCTET STRING</code>, which has the tag &ldquo;0x04&rdquo;. <code>OCTET STRING</code> means &ldquo;here&rsquo;s
a bunch of bytes!&rdquo; In this case, as described by the spec, those bytes
happen to contain more DER. This is a fairly common pattern in X.509
to deal with parameterized data. For instance, this allows defining a
structure for extensions without knowing ahead of time all the structures
that a future extension might want to carry in its value. If you&rsquo;re a C
programmer, think of it as a <code>void*</code> for data structures. If you prefer Go,
think of it as an <code>interface{}</code>.</p>

<p>Here&rsquo;s that <code>extnValue</code>:</p>

<pre><code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
</code></pre>

<p>That&rsquo;s tag &ldquo;0x04&rdquo;, meaning <code>OCTET STRING</code>, followed by &ldquo;0x81 0xF5&rdquo;, meaning
&ldquo;this string is 245 bytes long&rdquo; (the 0x81 prefix is part of <a href="#variable-length">variable length
number encoding</a>).</p>

<p>According to <a href="https://tools.ietf.org/html/rfc6962#section-3.3">RFC 6962, section
3.3</a>, &ldquo;obtained SCTs
can be directly embedded in the final certificate, by encoding the
SignedCertificateTimestampList structure as an ASN.1 <code>OCTET STRING</code>
and inserting the resulting data in the TBSCertificate as an X.509v3
certificate extension&rdquo;</p>

<p>So, we have an <code>OCTET STRING</code>, all&rsquo;s good, right? Except if you remove the
tag and length from extnValue to get its value, you&rsquo;re left with:</p>

<pre><code>04 81 F2 00F0007500DB74AFEEC…
</code></pre>

<p>There&rsquo;s that &ldquo;0x04&rdquo; tag again, but with a shorter length. Why
do we nest one <code>OCTET STRING</code> inside another? It&rsquo;s because the
contents of extnValue are required by RFC 5280 to be valid DER, but a
SignedCertificateTimestampList is not encoded using DER (more on that
in a minute). So, by RFC 6962, a SignedCertificateTimestampList is wrapped in an
<code>OCTET STRING</code>, which is wrapped in another <code>OCTET STRING</code> (the extnValue).</p>

<p>Once we decode that second <code>OCTET STRING</code>, we&rsquo;re left with the contents:</p>

<pre><code>00F0007500DB74AFEEC…
</code></pre>

<p>&ldquo;0x00&rdquo; isn&rsquo;t a valid tag in DER. What is this? It&rsquo;s TLS encoding. This is
defined in <a href="https://tools.ietf.org/html/rfc5246#section-4">RFC 5246, section 4</a>
(the TLS 1.2 RFC). TLS encoding, like ASN.1, has both a way to define data
structures and a way to encode those structures. TLS encoding differs
from DER in that there are no tags, and lengths are only encoded when necessary for
variable-length arrays. Within an encoded structure, the type of a field is determined by
its position, rather than by a tag. This means that TLS-encoded structures are
more compact than DER structures, but also that they can&rsquo;t be processed without
knowing the corresponding schema. For instance, here&rsquo;s the top-level schema from
<a href="https://tools.ietf.org/html/rfc6962#section-3.3">RFC 6962, section 3.3</a>:</p>

<pre><code> The contents of the ASN.1 OCTET STRING embedded in an OCSP extension
or X509v3 certificate extension are as follows:

opaque SerializedSCT&lt;1..2^16-1&gt;;

struct {
SerializedSCT sct_list &lt;1..2^16-1&gt;;
} SignedCertificateTimestampList;

Here, &quot;SerializedSCT&quot; is an opaque byte string that contains the
serialized TLS structure.
</code></pre>

<p>Right away, we&rsquo;ve found one of those variable-length arrays. The length of such
an array (in bytes) is always represented by a length field just big enough to
hold the max array size. The max size of an <code>sct_list</code> is 65535 bytes, so the
length field is two bytes wide. Sure enough, those first two bytes are &ldquo;0x00
0xF0&rdquo;, or 240 in decimal. In other words, this <code>sct_list</code> will have 240 bytes. We
don&rsquo;t yet know how many SCTs will be in it. That will become clear only by
continuing to parse the encoded data and seeing where each struct ends (spoiler
alert: there are two SCTs!).</p>

<p>Now we know the first SerializedSCT starts with <code>0075…</code>. SerializedSCT
is itself a variable-length field, this time containing <code>opaque</code> bytes (much like <code>OCTET STRING</code>
back in the ASN.1 world). Like SignedCertificateTimestampList, it has a max size
of 65535 bytes, so we pull off the first two bytes and discover that the first
SerializedSCT is 0x0075 (117 decimal) bytes long. Here&rsquo;s the whole thing, in
hex:</p>

<pre><code>00DB74AFEECB29ECB1FECA3E716D2CE5B9AABB36F7847183C75D9D4F37B61FBF64000001627313EB19000004030046304402207E1FCD1E9A2BD2A50A0C81E713033A0762340DA8F91EF27A48B3817640159CD30220659FE9F1D880E2E8F6B325BE9F18956D17C6CA8A6F2B12CB0F55FB70F759A419
</code></pre>

<p>This can be decoded using the TLS encoding struct defined in <a href="https://tools.ietf.org/html/rfc6962#page-13">RFC 6962, section
3.2</a>:</p>

<pre><code>enum { v1(0), (255) }
Version;

struct {
opaque key_id[32];
} LogID;

opaque CtExtensions&lt;0..2^16-1&gt;;

struct {
Version sct_version;
LogID id;
uint64 timestamp;
CtExtensions extensions;
digitally-signed struct {
Version sct_version;
SignatureType signature_type = certificate_timestamp;
uint64 timestamp;
LogEntryType entry_type;
select(entry_type) {
case x509_entry: ASN.1Cert;
case precert_entry: PreCert;
} signed_entry;
CtExtensions extensions;
};
} SignedCertificateTimestamp;
</code></pre>

<p>Breaking that down:</p>

<pre><code># Version sct_version v1(0)
00
# LogID id (aka opaque key_id[32])
DB74AFEECB29ECB1FECA3E716D2CE5B9AABB36F7847183C75D9D4F37B61FBF64
# uint64 timestamp (milliseconds since the epoch)
000001627313EB19
# CtExtensions extensions (zero-length array)
0000
# digitally-signed struct
04030046304402207E1FCD1E9A2BD2A50A0C81E713033A0762340DA8F91EF27A48B3817640159CD30220659FE9F1D880E2E8F6B325BE9F18956D17C6CA8A6F2B12CB0F55FB70F759A419
</code></pre>

<p>To understand the &ldquo;digitally-signed struct,&rdquo; we need to turn back to <a href="https://tools.ietf.org/html/rfc5246#section-4.7">RFC 5246,
section 4.7</a>. It says:</p>

<pre><code>A digitally-signed element is encoded as a struct DigitallySigned:

struct {
SignatureAndHashAlgorithm algorithm;
opaque signature&lt;0..2^16-1&gt;;
} DigitallySigned;
</code></pre>

<p>And in <a href="https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1">section
7.4.1.4.1</a>:</p>

<pre><code>enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;

enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) }
SignatureAlgorithm;

struct {
HashAlgorithm hash;
SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;
</code></pre>

<p>We have &ldquo;0x0403&rdquo;, which corresponds to sha256(4) and ecdsa(3). The next two
bytes, &ldquo;0x0046&rdquo;, tell us the length of the &ldquo;opaque signature&rdquo; field, 70 bytes in
decimal. To decode the signature, we reference <a href="https://tools.ietf.org/html/rfc4492#page-20">RFC 4492 section
5.4</a>, which says:</p>

<pre><code>The digitally-signed element is encoded as an opaque vector &lt;0..2^16-1&gt;, the
contents of which are the DER encoding corresponding to the
following ASN.1 notation.

Ecdsa-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER
}
</code></pre>

<p>Having dived through two layers of TLS encoding, we are now back in ASN.1 land!
We
<a href="https://lapo.it/asn1js/#304402207E1FCD1E9A2BD2A50A0C81E713033A0762340DA8F91EF27A48B3817640159CD30220659FE9F1D880E2E8F6B325BE9F18956D17C6CA8A6F2B12CB0F55FB70F759A419">decode</a>
the remaining bytes into a SEQUENCE containing two INTEGERS. And we&rsquo;re done! Here&rsquo;s the whole
extension decoded:</p>

<pre><code># Extension SEQUENCE – RFC 5280
30
# length 0x0104 bytes (260 decimal)
820104
# OBJECT IDENTIFIER
06
# length 0x0A bytes (10 decimal)
0A
# value (1.3.6.1.4.1.11129.2.4.2)
2B06010401D679020402
# OCTET STRING
04
# length 0xF5 bytes (245 decimal)
81F5
# OCTET STRING (embedded) – RFC 6962
04
# length 0xF2 bytes (242 decimal)
81F2
# Beginning of TLS encoded SignedCertificateTimestampList – RFC 5246 / 6962
# length 0xF0 bytes
00F0
# opaque SerializedSCT&lt;1..2^16-1&gt;
# length 0x75 bytes
0075
# Version sct_version v1(0)
00
# LogID id (aka opaque key_id[32])
DB74AFEECB29ECB1FECA3E716D2CE5B9AABB36F7847183C75D9D4F37B61FBF64
# uint64 timestamp (milliseconds since the epoch)
000001627313EB19
# CtExtensions extensions (zero-length array)
0000
# digitally-signed struct – RFC 5426
# SignatureAndHashAlgorithm (ecdsa-sha256)
0403
# opaque signature&lt;0..2^16-1&gt;;
# length 0x0046
0046
# DER-encoded Ecdsa-Sig-Value – RFC 4492
30 # SEQUENCE
44 # length 0x44 bytes
02 # r INTEGER
20 # length 0x20 bytes
# value
7E1FCD1E9A2BD2A50A0C81E713033A0762340DA8F91EF27A48B3817640159CD3
02 # s INTEGER
20 # length 0x20 bytes
# value
659FE9F1D880E2E8F6B325BE9F18956D17C6CA8A6F2B12CB0F55FB70F759A419
# opaque SerializedSCT&lt;1..2^16-1&gt;
# length 0x77 bytes
0077
# Version sct_version v1(0)
00
# LogID id (aka opaque key_id[32])
293C519654C83965BAAA50FC5807D4B76FBF587A2972DCA4C30CF4E54547F478
# uint64 timestamp (milliseconds since the epoch)
000001627313EB2A
# CtExtensions extensions (zero-length array)
0000
# digitally-signed struct – RFC 5426
# SignatureAndHashAlgorithm (ecdsa-sha256)
0403
# opaque signature&lt;0..2^16-1&gt;;
# length 0x0048
0048
# DER-encoded Ecdsa-Sig-Value – RFC 4492
30 # SEQUENCE
46 # length 0x46 bytes
02 # r INTEGER
21 # length 0x21 bytes
# value
00AB72F1E4D6223EF87FC68491C208D29D4D57EBF47588BB7544D32F9537E2CEC1
02 # s INTEGER
21 # length 0x21 bytes
# value
008AFFC40CC6C4E3B24578DADE4F815ECBCE2D57A579342119A1E65BC7E5E69CE2
</code></pre>

<p>One surprising thing you might notice: In the first SCT, <code>r</code> and <code>s</code> are twenty
bytes long. In the second SCT, they are both twenty-one bytes long, and have a
leading zero. Integers in DER are two&rsquo;s complement, so if the leftmost bit is
set, they are interpreted as negative. Since <code>r</code> and <code>s</code> are positive, if the
leftmost bit would be a 1, an extra byte has to be added so that the leftmost
bit can be 0.</p>

<p>This is a little taste of what goes into encoding a certificate. I hope it was
informative! If you&rsquo;d like to learn more, I recommend &ldquo;<a href="http://luca.ntop.org/Teaching/Appunti/asn1.html">A Layman&rsquo;s Guide to a
Subset of ASN.1, BER, and DER</a>.&rdquo;</p>

<p><a name="poison"></a>Footnote 1: A &ldquo;poison extension&rdquo; is defined by <a href="https://tools.ietf.org/html/rfc6962#section-3.1">RFC 6962
section 3.1</a>:</p>

<pre><code>The Precertificate is constructed from the certificate to be issued by adding a special
critical poison extension (OID `1.3.6.1.4.1.11129.2.4.3`, whose
extnValue OCTET STRING contains ASN.1 NULL data (0x05 0x00))
</code></pre>

<p>In other words, it&rsquo;s an empty extension whose only purpose is to ensure that
certificate processors will not accept precertificates as valid certificates. The
specification ensures this by setting the &ldquo;critical&rdquo; bit on the extension, which
ensures that code that doesn&rsquo;t recognize the extension will reject the whole
certificate. Code that does recognize the extension specifically as poison
will also reject the certificate.</p>

<p><a name="variable-length"></a>Footnote 2: Lengths from 0-127 are represented by
a single byte (short form). To express longer lengths, more bytes are used (long form).
The high bit (0x80) on the first byte is set to distinguish long form from short
form. The remaining bits are used to express how many more bytes to read for the
length. For instance, 0x81F5 means &ldquo;this is long form because the length is
greater than 127, but there&rsquo;s still only one byte of length (0xF5) to decode.&rdquo;</p>

2017-11-06 задача

Post Syndicated from Vasil Kolev original https://vasil.ludost.net/blog/?p=3368

(по-подробно за феста – като се наспя)

За OpenFest 2017 за щанда на StorPool бях написал една задача, та който я реши, да получи тениска. Задачата звучи измамно просто и аз също не съм се усетил, че не е лесно решима за 10 минути.

Задачата е следната – имате директория с някакво количество файлове, да видите кои от тях са MD5 и кои – SHA1 колизии, и да дадете първите букви от имената им (4 файла за md5 и 4 за sha1). Моето решение беше във временна директория да се направят файлове с имена MD5 (и после – SHA1) сумите, в които да се напишат имената и SHA256 сумите на файловете с тая MD5 сума, и после с един sort на всеки файл лесно се вижда в кой има различни файлове (трябва да са еднакви по принцип). Ако е просто да се види коя е md5 сумата, може да се броят уникалните sha256 суми във всички файлове, да се види къде са колизиите.

Интересно ще ми е наистина ли е толкова трудна задачата (доколкото знам, за два дни само един човек я е решил за 10 минути).

Също така ми е интересно дали някой не е решил да пита google какви са checksum-ите на демонстрационните sha1/md5 колизии и да види дали аз не съм си събрал файловете по тоя начин…

Кодът, който генерира задачата е качен на https://vasil.ludost.net/progs/storpool-of-task.tgz. Вътре има gen.sh, който трябва да се пипне малко къде да прави файловете и който при пускане създава малко файлове и ви дава отговора. Не съм сложил другите неща (това, което се прави на login shell и нещото, което праща отговорите по slack на проверяващия), но те не са толкова интересни.

Application Load Balancers Now Support Multiple TLS Certificates With Smart Selection Using SNI

Post Syndicated from Randall Hunt original https://aws.amazon.com/blogs/aws/new-application-load-balancer-sni/

Today we’re launching support for multiple TLS/SSL certificates on Application Load Balancers (ALB) using Server Name Indication (SNI). You can now host multiple TLS secured applications, each with its own TLS certificate, behind a single load balancer. In order to use SNI, all you need to do is bind multiple certificates to the same secure listener on your load balancer. ALB will automatically choose the optimal TLS certificate for each client. These new features are provided at no additional charge.

If you’re looking for a TL;DR on how to use this new feature just click here. If you’re like me and you’re a little rusty on the specifics of Transport Layer Security (TLS) then keep reading.

TLS? SSL? SNI?

People tend to use the terms SSL and TLS interchangeably even though the two are technically different. SSL technically refers to a predecessor of the TLS protocol. To keep things simple I’ll be using the term TLS for the rest of this post.

TLS is a protocol for securely transmitting data like passwords, cookies, and credit card numbers. It enables privacy, authentication, and integrity of the data being transmitted. TLS uses certificate based authentication where certificates are like ID cards for your websites. You trust the person that signed and issued the certificate, the certificate authority (CA), so you trust that the data in the certificate is correct. When a browser connects to your TLS-enabled ALB, ALB presents a certificate that contains your site’s public key, which has been cryptographically signed by a CA. This way the client can be sure it’s getting the ‘real you’ and that it’s safe to use your site’s public key to establish a secure connection.

With SNI support we’re making it easy to use more than one certificate with the same ALB. The most common reason you might want to use multiple certificates is to handle different domains with the same load balancer. It’s always been possible to use wildcard and subject-alternate-name (SAN) certificates with ALB, but these come with limitations. Wildcard certificates only work for related subdomains that match a simple pattern and while SAN certificates can support many different domains, the same certificate authority has to authenticate each one. That means you have reauthenticate and reprovision your certificate everytime you add a new domain.

One of our most frequent requests on forums, reddit, and in my e-mail inbox has been to use the Server Name Indication (SNI) extension of TLS to choose a certificate for a client. Since TLS operates at the transport layer, below HTTP, it doesn’t see the hostname requested by a client. SNI works by having the client tell the server “This is the domain I expect to get a certificate for” when it first connects. The server can then choose the correct certificate to respond to the client. All modern web browsers and a large majority of other clients support SNI. In fact, today we see SNI supported by over 99.5% of clients connecting to CloudFront.

Smart Certificate Selection on ALB

ALB’s smart certificate selection goes beyond SNI. In addition to containing a list of valid domain names, certificates also describe the type of key exchange and cryptography that the server supports, as well as the signature algorithm (SHA2, SHA1, MD5) used to sign the certificate. To establish a TLS connection, a client starts a TLS handshake by sending a “ClientHello” message that outlines the capabilities of the client: the protocol versions, extensions, cipher suites, and compression methods. Based on what an individual client supports, ALB’s smart selection algorithm chooses a certificate for the connection and sends it to the client. ALB supports both the classic RSA algorithm and the newer, hipper, and faster Elliptic-curve based ECDSA algorithm. ECDSA support among clients isn’t as prevalent as SNI, but it is supported by all modern web browsers. Since it’s faster and requires less CPU, it can be particularly useful for ultra-low latency applications and for conserving the amount of battery used by mobile applications. Since ALB can see what each client supports from the TLS handshake, you can upload both RSA and ECDSA certificates for the same domains and ALB will automatically choose the best one for each client.

Using SNI with ALB

I’ll use a few example websites like VimIsBetterThanEmacs.com and VimIsTheBest.com. I’ve purchased and hosted these domains on Amazon Route 53, and provisioned two separate certificates for them in AWS Certificate Manager (ACM). If I want to securely serve both of these sites through a single ALB, I can quickly add both certificates in the console.

First, I’ll select my load balancer in the console, go to the listeners tab, and select “view/edit certificates”.

Next, I’ll use the “+” button in the top left corner to select some certificates then I’ll click the “Add” button.

There are no more steps. If you’re not really a GUI kind of person you’ll be pleased to know that it’s also simple to add new certificates via the AWS Command Line Interface (CLI) (or SDKs).

aws elbv2 add-listener-certificates --listener-arn <listener-arn> --certificates CertificateArn=<cert-arn>

Things to know

  • ALB Access Logs now include the client’s requested hostname and the certificate ARN used. If the “hostname” field is empty (represented by a “-“) the client did not use the SNI extension in their request.
  • You can use any of your certificates in ACM or IAM.
  • You can bind multiple certificates for the same domain(s) to a secure listener. Your ALB will choose the optimal certificate based on multiple factors including the capabilities of the client.
  • If the client does not support SNI your ALB will use the default certificate (the one you specified when you created the listener).
  • There are three new ELB API calls: AddListenerCertificates, RemoveListenerCertificates, and DescribeListenerCertificates.
  • You can bind up to 25 certificates per load balancer (not counting the default certificate).
  • These new features are supported by AWS CloudFormation at launch.

You can see an example of these new features in action with a set of websites created by my colleague Jon Zobrist: https://www.exampleloadbalancer.com/.

Overall, I will personally use this feature and I’m sure a ton of AWS users will benefit from it as well. I want to thank the Elastic Load Balancing team for all their hard work in getting this into the hands of our users.

Randall

HashPump – Exploit Hash Length Extension Attack

Post Syndicated from Darknet original http://feedproxy.google.com/~r/darknethackers/~3/3DOE2xyGowM/

HashPump is a C++ based command line tool to exploit the Hash Length Extension Attack with various hash types supported, including MD4, MD5, SHA1, SHA256, and SHA512. There’s a good write-up of how to use this in practical terms here: Plaid CTF 2014: mtpox Usage [crayon-58d9345a724a6910508053/] You can download HashPump here:…

Read the full post at darknet.org.uk

New AWS Encryption SDK for Python Simplifies Multiple Master Key Encryption

Post Syndicated from Matt Bullock original https://aws.amazon.com/blogs/security/new-aws-encryption-sdk-for-python-simplifies-multiple-master-key-encryption/

The AWS Cryptography team is happy to announce a Python implementation of the AWS Encryption SDK. This new SDK helps manage data keys for you, and it simplifies the process of encrypting data under multiple master keys. As a result, this new SDK allows you to focus on the code that drives your business forward. It also provides a framework you can easily extend to ensure that you have a cryptographic library that is configured to match and enforce your standards. The SDK also includes ready-to-use examples. If you are a Java developer, you can refer to this blog post to see specific Java examples for the SDK.

In this blog post, I show you how you can use the AWS Encryption SDK to simplify the process of encrypting data and how to protect your encryption keys in ways that help improve application availability by not tying you to a single region or key management solution.

How does the AWS Encryption SDK help me?

Developers using encryption often face three problems:

  1. How do I correctly generate and use a data key to encrypt data?
  2. How do I protect the data key after it has been used?
  3. How do I store the data key and ciphertext in a portable manner?

The library provided in the AWS Encryption SDK addresses the first problem by implementing the low-level envelope encryption details transparently using the cryptographic provider available in your development environment. The library helps address the second problem by providing intuitive interfaces to let you choose how you want to generate data keys and the master keys or key-encrypting keys that will protect data keys. Developers can then focus on the core of the application they are building instead of on the complexities of encryption. The ciphertext addresses the third problem, as described later in this post.

The AWS Encryption SDK defines a carefully designed and reviewed ciphertext data format that supports multiple secure algorithm combinations (with room for future expansion) and has no limits on the types or algorithms of the master keys. The ciphertext output of clients (created with the SDK) is a single binary blob that contains your encrypted message and one or more copies of the data key, as encrypted by each master key referenced in the encryption request. This single ciphertext data format for envelope-encrypted data makes it easier to ensure the data key has the same durability and availability properties as the encrypted message itself.

The AWS Encryption SDK provides production-ready reference implementations in Java and Python with direct support for key providers such as AWS Key Management Service (KMS). The Java implementation also supports the Java Cryptography Architecture (JCA/JCE) natively, which includes support for AWS CloudHSM and other PKCS #11 devices. The standard ciphertext data format the AWS Encryption SDK defines means that you can use combinations of the Java and Python clients for encryption and decryption as long as they each have access to the key provider that manages the correct master key used to encrypt the data key.

Let’s look at how you can use the AWS Encryption SDK to simplify the process of encrypting data and how to protect your data keys in ways that help improve application availability by not tying you to a single region or key management solution.

Example 1: Encrypting application secrets under multiple regional KMS master keys for high availability

Many customers want to build systems that not only span multiple Availability Zones, but also multiple regions. You cannot share KMS customer master keys (CMKs) across regions. However, with envelope encryption, you can encrypt the data key with multiple KMS CMKs in different regions. Applications running in each region can use the local KMS endpoint to decrypt the ciphertext for faster and more reliable access.

For the examples in this post, I will assume that I am running on Amazon EC2 instances configured with IAM roles for EC2. This enables me to avoid credential management and take advantage of built-in logic that routes requests to the nearest endpoints. These examples also assume that the latest version of the AWS SDK for Python (different from the AWS Encryption SDK) is available.

The encryption logic has a simple high-level design. Using provided parameters, I get the master keys and use them to encrypt some provided data, as shown in the following code example. I will define how to construct the multi-region KMS key provider next.

import aws_encryption_sdk


def encrypt_data(plaintext):
    # Get all the master keys needed
    key_provider = build_multiregion_kms_master_key_provider()

    # Encrypt the provided data
    ciphertext, header = aws_encryption_sdk.encrypt(
        source=plaintext,
        key_provider=key_provider
    )
    return ciphertext

Create a master key provider containing multiple master keys

The following code example shows how you can encrypt data under CMKs in three US regions: us-east-1, us-west-1, and us-west-2. The example assumes that you have already set up the CMKs and created an alias named alias/exampleKey in each region for each CMK. For more information about creating CMKs and aliases, see Creating Keys in the AWS KMS documentation.

This example creates a single KMSMasterKeyProvider to which all CMKs are added. The KMSMasterKeyProvider handles interacting with CMKs in multiple regions. Note that the first master key added to the KMSMasterKeyProvider is the one used to generate the new data key, and the other master keys are used to encrypt the new data key.

import aws_encryption_sdk
import boto3


def build_multiregion_kms_master_key_provider():
    regions = ('us-east-1', 'us-west-1', 'us-west-2')
    alias = 'alias/exampleKey'
    arn_template = 'arn:aws:kms:{region}:{account_id}:{alias}'

    # Create AWS KMS master key provider
    kms_master_key_provider = aws_encryption_sdk.KMSMasterKeyProvider()

    # Find your AWS account ID
    account_id = boto3.client('sts').get_caller_identity()['Account']

    # Add the KMS alias in each region to the master key provider
    for region in regions:
        kms_master_key_provider.add_master_key(arn_template.format(
            region=region,
            account_id=account_id,
            alias=alias
        ))
    return kms_master_key_provider

The logic to construct a master key provider could be built once by your central security team and then reused across your company to both simplify development and ensure that all encrypted data meets corporate standards.

Encrypt the data

The data you encrypt can come from anywhere and you can distribute it however you like. In the following code example, I read a file from disk and write out an encrypted copy. The AWS Encryption SDK provides a stream interface that behaves as a standard Python stream context manager to make this easy.

import aws_encryption_sdk
import boto3


def encrypt_file(input_filename, output_filename):
    # Get all the master keys needed
    key_provider = build_multiregion_kms_master_key_provider()

    # Open the files for reading and writing
    with open(input_filename, 'rb') as infile,\
            open(output_filename, 'wb') as outfile:
        # Encrypt the file
        with aws_encryption_sdk.stream(
            mode='e',
            source=infile,
            key_provider=key_provider
        ) as encryptor:
            for chunk in encryptor:
                outfile.write(chunk)

This file could contain, for example, secret application configuration data (such as passwords, certificates, and the like) that is then sent to EC2 instances as EC2 user data upon launch.

Decrypt the data

The following code example decrypts the contents of the EC2 user data and writes it to the specified file. The KMSMasterKeyProvider  defaults to using KMS in the local region, so decryption proceeds quickly without cross-region calls.

from botocore.vendored import requests


def decrypt_user_data(output_filename):
    # Create a master key provider that points to the local KMS stack
    kms_key_provider = aws_encryption_sdk.KMSMasterKeyProvider()

    # Read the user data
    user_data = requests.get('http://169.254.169.254/latest/user-data/').content
    # Open a stream to write out the decrypted file
    # Decrypt the userdata and write the plaintext into the file
    with open(output_filename, 'wb') as outfile,\
            aws_encryption_sdk.stream(
                mode='d',
                source=user_data,
                key_provider=kms_key_provider
            ) as decryptor:
        for chunk in decryptor:
            outfile.write(chunk)

Congratulations! You have just encrypted data under master keys in multiple regions and have code that will always decrypt the data by using the local KMS stack. This gives you higher availability and lower latency for decryption, while still only needing to manage a single ciphertext.

Example 2: Encrypting application secrets under master keys from different providers for escrow and portability

Another reason why you might want to encrypt data under multiple master keys is to avoid relying on a single provider for your keys. By not tying yourself to a single key management solution, you help improve your applications’ availability. This approach also might help if you have compliance, data loss prevention, or disaster recovery requirements that require multiple providers.

You can use the same technique demonstrated previously in this post to encrypt your data to an escrow or additional decryption master key that is independent of your primary provider. This example demonstrates how to use an additional master key, which is an RSA public key randomly generated upon request. (Storing and managing the RSA key pair are out of scope for this blog.)

Encrypt the data with a public master key

Just like the previous code example that created a number of KMS master keys to encrypt data, the following code example creates one more master key for use with the RSA public key.

import aws_encryption_sdk
from aws_encryption_sdk.internal.crypto import WrappingKey
from aws_encryption_sdk.key_providers.raw import RawMasterKeyProvider
from aws_encryption_sdk.identifiers import WrappingAlgorithm, EncryptionKeyType
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa


class StaticRandomMasterKeyProvider(RawMasterKeyProvider):
    """Randomly generates and provides 4096-bit RSA keys consistently per unique key id."""
    provider_id = 'static-random'

    def __init__(self, **kwargs):
        self._static_keys = {}

    def _get_raw_key(self, key_id):
        """Retrieves a static, randomly generated RSA key for the specified key id.

        :param str key_id: Key ID
        :returns: Wrapping key which contains the specified static key
        :rtype: :class:`aws_encryption_sdk.internal.crypto.WrappingKey`
        """
        try:
            static_key = self._static_keys[key_id]
        except KeyError:
            private_key = rsa.generate_private_key(
                public_exponent=65537,
                key_size=4096,
                backend=default_backend()
            )
            static_key = private_key.private_bytes(
                encoding=serialization.Encoding.PEM,
                format=serialization.PrivateFormat.PKCS8,
                encryption_algorithm=serialization.NoEncryption()
            )
            self._static_keys[key_id] = static_key
        return WrappingKey(
            wrapping_algorithm=WrappingAlgorithm.RSA_OAEP_SHA1_MGF1,
            wrapping_key=static_key,
            wrapping_key_type=EncryptionKeyType.PRIVATE
        )


def get_multi_master_key_provider():
    # Create multiregion KMS master key provider
    multi_master_key_provider = build_multiregion_kms_master_key_provider()

    # Create static master key provider and add a key
    static_key_id = os.urandom(8)
    static_master_key_provider = StaticRandomMasterKeyProvider()
    static_master_key_provider.add_master_key(static_key_id)

    # Add static master key provider to KMS master key provider
    multi_master_key_provider.add_master_key_provider(static_master_key_provider)

    return multi_master_key_provider, static_master_key_provider

Decrypt the data with the private key

The following decryption code example uses the static RSA master key provider generated previously to demonstrate decryption with a non-AWS master key.

def cycle_data(input_data):
    # Create multi-source master key provider
    multi_master_key_provider, static_master_key_provider = get_multi_master_key_provider()

    # Encrypt data with multi-source master key provider
    ciphertext, header = aws_encryption_sdk.encrypt(
        source=input_data,
        key_provider=multi_master_key_provider
    )

    # Decrypt data using only static master key provider
    plaintext, header = aws_encryption_sdk.decrypt(
        source=ciphertext,
        key_provider=static_master_key_provider
    )

Conclusion

Envelope encryption is powerful, but traditionally, it has been challenging to implement. The new AWS Encryption SDK helps manage data keys for you, and it simplifies the process of encrypting data under multiple master keys. As a result, this new SDK allows you to focus on the code that drives your business forward. It also provides a framework you can easily extend to ensure that you have a cryptographic library that is configured to match and enforce your standards.

We are excited about releasing the AWS Encryption SDK and cannot wait to hear what you do with it. If you have comments about the new SDK or anything in this blog post, submit a comment in the “Comments” section below. If you have implementation or usage questions, start a new thread on the KMS forum.

– Matt

Subversion SHA1 collision problem statement

Post Syndicated from corbet original https://lwn.net/Articles/715873/rss

Users of the Subversion source-code management system may want to take a
look at this
post from Mark Phippard
. He explains how hash collisions can corrupt a
repository and a couple of short-term workarounds. “The quick
summary if you do not want to read this entire post is that the problem is
really not that bad. If you run into it there are solutions to resolve it
and you are not going to run into it in normal usage. There will also
likely be some future updates to Subversion that avoid it entirely so if
you regularly update your server and client when new releases come out you
are probably safe not doing anything and just waiting for an update to
happen.

SHA-1 Collision Found

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2017/02/sha-1_collision.html

The first collision in the SHA-1 hash function has been found.

This is not a surprise. We’ve all expected this for over a decade, watching computing power increase. This is why NIST standardized SHA-3 in 2012.

EDITED TO ADD (2/24): Website for the collision. (Yes, this brute-force example has its own website.)