tech0006.txt

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Tech Note 0006
PK Standards Compliance
Tom St Denis

RSA
----

PKCS #1 compliance.

Key Format:  RSAPublicKey and RSAPrivateKey as per PKCS #1 v2.1
Encryption:  OAEP as per PKCS #1
Signature :  PSS  as per PKCS #1

DSA
----

The NIST DSA algorithm

Key Format:  HomeBrew [see below]
Signature :  ANSI X9.62 format [see below].

Keys are stored as 

DSAPublicKey ::= SEQUENCE {
    publicFlags    BIT STRING(1), -- must be 0
    g              INTEGER      , -- base generator, check that g^q mod p == 1
                                  -- and that 1 < g < p - 1
    p              INTEGER      , -- prime modulus 
    q              INTEGER      , -- order of sub-group (must be prime)
    y              INTEGER      , -- public key, specifically, g^x mod p, 
                                  -- check that y^q mod p == 1
                                  -- and that 1 < y < p - 1
}

DSAPrivateKey ::= SEQUENCE {
    publicFlags    BIT STRING(1), -- must be 1
    g              INTEGER      , -- base generator, check that g^q mod p == 1
                                  -- and that 1 < g < p - 1
    p              INTEGER      , -- prime modulus 
    q              INTEGER      , -- order of sub-group (must be prime)
    y              INTEGER      , -- public key, specifically, g^x mod p, 
                                  -- check that y^q mod p == 1
                                  -- and that 1 < y < p - 1
    x              INTEGER        -- private key
}

Signatures are stored as 

DSASignature ::= SEQUENCE {
    r, s           INTEGER        -- signature parameters
}

ECC
----

The ANSI X9.62 and X9.63 algorithms [partial].  Supports all NIST GF(p) curves.

Key Format   :  Homebrew [see below, only GF(p) NIST curves supported]
Signature    :  X9.62 compliant
Encryption   :  Homebrew [based on X9.63, differs in that the public point is stored as an ECCPublicKey]
Shared Secret:  X9.63 compliant

ECCPublicKey ::= SEQUENCE {
    flags       BIT STRING(1), -- public/private flag (always zero), 
    keySize     INTEGER,       -- Curve size (in bits) divided by eight 
                               -- and rounded down, e.g. 521 => 65
    pubkey.x    INTEGER,       -- The X co-ordinate of the public key point
    pubkey.y    INTEGER,       -- The Y co-ordinate of the public key point
}

ECCPrivateKey ::= SEQUENCE {
    flags       BIT STRING(1), -- public/private flag (always one), 
    keySize     INTEGER,       -- Curve size (in bits) divided by eight 
                               -- and rounded down, e.g. 521 => 65
    pubkey.x    INTEGER,       -- The X co-ordinate of the public key point
    pubkey.y    INTEGER,       -- The Y co-ordinate of the public key point
    secret.k    INTEGER,       -- The secret key scalar
}

The encryption works by finding the X9.63 shared secret and hashing it.  The hash is then simply XOR'ed against the message [which must be at most the size
of the hash digest].  The format of the encrypted text is as follows

ECCEncrypted ::= SEQUENCE {
    hashOID     OBJECT IDENTIFIER,   -- The OID of the hash used
    pubkey      OCTET STRING     ,   -- Encapsulation of a random ECCPublicKey
    skey        OCTET STRING         -- The encrypted text (which the hash was XOR'ed against)
}

% $Source: /cvs/libtom/libtomcrypt/notes/tech0006.txt,v $   
% $Revision: 1.2 $   
% $Date: 2005/06/18 02:26:27 $