This removes a bunch of weird code around the counter overflow check in
concatKDF and makes it actually work for different hash output sizes.
The overflow check worked as follows: concatKDF applies the hash function N
times, where N is roundup(kdLen, hashsize) / hashsize. N should not
overflow 32 bits because that would lead to a repetition in the KDF output.
A couple issues with the overflow check:
- It used the hash.BlockSize, which is wrong because the
block size is about the input of the hash function. Luckily, all standard
hash functions have a block size that's greater than the output size, so
concatKDF didn't crash, it just generated too much key material.
- The check used big.Int to compare against 2^32-1.
- The calculation could still overflow before reaching the check.
The new code in concatKDF doesn't check for overflow. Instead, there is a
new check on ECIESParams which ensures that params.KeyLen is < 512. This
removes any possibility of overflow.
There are a couple of miscellaneous improvements bundled in with this
change:
- The key buffer is pre-allocated instead of appending the hash output
to an initially empty slice.
- The code that uses concatKDF to derive keys is now shared between Encrypt
and Decrypt.
- There was a redundant invocation of IsOnCurve in Decrypt. This is now removed
because elliptic.Unmarshal already checks whether the input is a valid curve
point since Go 1.5.
Co-authored-by: Felix Lange <fjl@twurst.com>
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# NOTE
This implementation is direct fork of Kylom's implementation. I claim no authorship over this code apart from some minor modifications.
Please be aware this code **has not yet been reviewed**.
ecies implements the Elliptic Curve Integrated Encryption Scheme.
The package is designed to be compliant with the appropriate NIST
standards, and therefore doesn't support the full SEC 1 algorithm set.
STATUS:
ecies should be ready for use. The ASN.1 support is only complete so
far as to supported the listed algorithms before.
CAVEATS
1. CMAC support is currently not present.
SUPPORTED ALGORITHMS
SYMMETRIC CIPHERS HASH FUNCTIONS
AES128 SHA-1
AES192 SHA-224
AES256 SHA-256
SHA-384
ELLIPTIC CURVE SHA-512
P256
P384 KEY DERIVATION FUNCTION
P521 NIST SP 800-65a Concatenation KDF
Curve P224 isn't supported because it does not provide a minimum security
level of AES128 with HMAC-SHA1. According to NIST SP 800-57, the security
level of P224 is 112 bits of security. Symmetric ciphers use CTR-mode;
message tags are computed using HMAC-<HASH> function.
CURVE SELECTION
According to NIST SP 800-57, the following curves should be selected:
+----------------+-------+
| SYMMETRIC SIZE | CURVE |
+----------------+-------+
| 128-bit | P256 |
+----------------+-------+
| 192-bit | P384 |
+----------------+-------+
| 256-bit | P521 |
+----------------+-------+
TODO
1. Look at serialising the parameters with the SEC 1 ASN.1 module.
2. Validate ASN.1 formats with SEC 1.
TEST VECTORS
The only test vectors I've found so far date from 1993, predating AES
and including only 163-bit curves. Therefore, there are no published
test vectors to compare to.
LICENSE
ecies is released under the same license as the Go source code. See the
LICENSE file for details.
REFERENCES
* SEC (Standard for Efficient Cryptography) 1, version 2.0: Elliptic
Curve Cryptography; Certicom, May 2009.
http://www.secg.org/sec1-v2.pdf
* GEC (Guidelines for Efficient Cryptography) 2, version 0.3: Test
Vectors for SEC 1; Certicom, September 1999.
http://read.pudn.com/downloads168/doc/772358/TestVectorsforSEC%201-gec2.pdf
* NIST SP 800-56a: Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography. National Institute of Standards
and Technology, May 2007.
http://csrc.nist.gov/publications/nistpubs/800-56A/SP800-56A_Revision1_Mar08-2007.pdf
* Suite B Implementer’s Guide to NIST SP 800-56A. National Security
Agency, July 28, 2009.
http://www.nsa.gov/ia/_files/SuiteB_Implementer_G-113808.pdf
* NIST SP 800-57: Recommendation for Key Management – Part 1: General
(Revision 3). National Institute of Standards and Technology, July
2012.
http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57_part1_rev3_general.pdf
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