GH355: Implement HKDF

This patch implements the HMAC-based Extract-and-Expand Key Derivation
Function (HKDF) as defined in RFC 5869.

It is required to implement the QUIC and TLS 1.3 protocols (among others).

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Dr. Stephen Henson <steve@openssl.org>

1 files changed, 103 insertions, 0 deletions

diff --git a/doc/crypto/EVP_PKEY_HKDF.pod b/doc/crypto/EVP_PKEY_HKDF.pod
new file mode 100644
index 0000000000..d44e130850
--- /dev/null
+++ b/doc/crypto/EVP_PKEY_HKDF.pod
@@ -0,0 +1,103 @@
+=pod
+
+=head1 NAME
+
+EVP_PKEY_HKDF; EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,
+EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info -
+HMAC-based Extract-and-Expand key derivation algorithm
+
+=head1 SYNOPSIS
+
+ #include <openssl/kdf.h>
+
+ int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
+
+ int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
+                                 int saltlen);
+
+ int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
+                                int keylen);
+
+ int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
+                                 int infolen);
+
+=head1 DESCRIPTION
+
+The EVP_PKEY_HKDF alogorithm implements the HKDF key derivation function.
+HKDF follows the "extract-then-expand" paradigm, where the KDF logically
+consists of two modules. The first stage takes the input keying material
+and "extracts" from it a fixed-length pseudorandom key K. The second stage
+"expands" the key K into several additional pseudorandom keys (the output
+of the KDF).
+
+EVP_PKEY_set_hkdf_md() sets the message digest associated with the HKDF.
+
+EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to B<saltlen> bytes of the
+buffer B<salt>. Any existing value is replaced.
+
+EVP_PKEY_CTX_set_hkdf_key() sets the key to B<keylen> bytes of the buffer
+B<key>. Any existing value is replaced.
+
+EVP_PKEY_CTX_add1_hkdf_info() sets the info value to B<infolen> bytes of the
+buffer B<info>. If a value is already set, it is appended to the existing
+value.
+
+=head1 NOTES
+
+All these functions are implemented as macros.
+
+A context for HKDF can be obtained by calling:
+
+ EVP_PKEY_CTX *pctx = EVP_PKEY_new_id(EVP_PKEY_HKDF, NULL);
+
+The digest, key, salt and info values must be set before a key is derived or
+an error occurs.
+
+The total length of the info buffer cannot exceed 1024 bytes in length: this
+should be more than enough for any normal use of HKDF.
+
+The output length of the KDF is specified by the length parameter in the
+EVP_PKEY_derive() function. Since the output length is variable, setting
+the buffer to B<NULL> is not meaningful for HKDF.
+
+Optimised versions of HKDF can be implemented in an ENGINE.
+
+=head1 RETURN VALUES
+
+All these functions return 1 for success and 0 or a negative value for failure.
+In particular a return value of -2 indicates the operation is not supported by
+the public key algorithm.
+
+=head1 EXAMPLE
+
+This example derives 10 bytes using SHA-256 with the secret key "secret",
+salt value "salt" and info value "label":
+
+ EVP_PKEY_CTX *pctx;
+ unsigned char out[10];
+ size_t outlen = sizeof(out);
+ pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
+
+ if (EVP_PKEY_derive_init(pctx) <= 0)
+    /* Error */
+ if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
+    /* Error */
+ if (EVP_PKEY_CTX_set1_salt(pctx, "salt", 4) <= 0)
+    /* Error */
+ if (EVP_PKEY_CTX_set1_key(pctx, "secret", 6) <= 0)
+    /* Error */
+ if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 6) <= 0)
+    /* Error */
+ if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
+    /* Error */
+
+=head1 CONFORMING TO
+
+RFC 5869
+
+=head1 SEE ALSO
+
+L<EVP_PKEY_CTX_new(3)>,
+L<EVP_PKEY_derive(3)>,
+
+=cut