This Attribute indicates the password of the user to be
authenticated, or the user's input following an Access-Challenge.
It is only used in Access-Request packets.
On transmission, the password is hidden. The password is first
padded at the end with nulls to a multiple of 16 octets. A one-
way MD5 hash is calculated over a stream of octets consisting of
the shared secret followed by the Request Authenticator. This
value is XORed with the first 16 octet segment of the password and
placed in the first 16 octets of the String field of the User-
If the password is longer than 16 characters, a second one-way MD5
hash is calculated over a stream of octets consisting of the
shared secret followed by the result of the first xor. That hash
is XORed with the second 16 octet segment of the password and
placed in the second 16 octets of the String field of the User-
If necessary, this operation is repeated, with each xor result
being used along with the shared secret to generate the next hash
to xor the next segment of the password, to no more than 128
The method is taken from the book "Network Security" by Kaufman,
Perlman and Speciner pages 109-110. A more precise
explanation of the method follows:
Call the shared secret S and the pseudo-random 128-bit Request
Authenticator RA. Break the password into 16-octet chunks p1, p2,
etc. with the last one padded at the end with nulls to a 16-octet
boundary. Call the ciphertext blocks c(1), c(2), etc. We'll need
intermediate values b1, b2, etc.
b1 = MD5(S + RA) c(1) = p1 xor b1
b2 = MD5(S + c(1)) c(2) = p2 xor b2
bi = MD5(S + c(i-1)) c(i) = pi xor bi
The String will contain c(1)+c(2)+...+c(i) where + denotes
concatenation.On receipt, the process is reversed to yield the original
A summary of the User-Password Attribute format is shown below. The
fields are transmitted from left to right.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length | String ...
2 for User-Password.
At least 18 and no larger than 130.
The String field is between 16 and 128 octets long, inclusive.