From: Stephan Somogyi <somogyi@digmedia.com>
Date: Fri, 29 Sep 95 12:34:44 PDT
To: cypherpunks@toad.com
Subject: STT is 40-bit for RC4, 56-bit for DES
Message-ID: <v03003502ac91f470ee63@[198.93.25.66]>
MIME-Version: 1.0
Content-Type: text/plain


From the STT spec, page 73:

6.8 CRYPTOGRAPHY

A. Encryption

Two bulk encryption algorithms are used in STT - RC4 and DES.

1. STT uses RC4 encryption with 8-byte keys, of which 3 bytes are salt,
in the clear. See the RC4Key entry under the Low Level Composites
sub-section of this document. RC4 is a stream cipher; there are no pad
bytes and the encrypted data is the same size as the plaintext data.

2. STT uses the Cipher Block Chaining (CBC) mode of DES, as defined in
Federal Information Processing Standard FIPS 81. The key is 8 bytes
long, with each byte having a parity bit in position 0. Thus there are
56 bits of random key. STT uses an all-zero byte Initialization Vector
(IV). A maximum of 8 bytes of padding is applied to every plaintext
message encrypted with DES to pad the message to a length that is a
multiple of 8 bytes. Pad bytes have a value of

      x = 8 - ((length of the plaintext) mod 8)

and the number of pad bytes is also x. For example, if the plaintext
message was 17 bytes long, then each of the 7 bytes of padding contains
the value 0x07. If x is 0, then there are 8 bytes, each containing
0x08. Padding is appended to the end of the plaintext before encryption
and is stripped off after decryption.

B. Signatures

STT uses PKCS #1 Encryption block formatting for RSA signatures. Total
length is 128 bytes for the signature (1024-bit modulus). The following
is the plaintext:

(TLV_SIGNATURE
    (BYTE[20] HashOfData)    ;Hash of the data being signed
    (BYTE 0)                 ;parser initializer
    (BYTE[105] 0xff)         ;padding
    (BYTE 0x01)              ;recom. for private key encryptions
    (BYTE 0))                ;overflow protection for RSA

C. Hashing

All hashes in STT are 20-byte SHA hashes. See Federal Information
Processing Standards FIPS 181 for the specification of SHA hashes.

________________________________________________________________________
Stephan Somogyi                Mr Gyroscope                Digital Media