RADIUS Attribute - User Password

As per RFC 2865

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-Password Attribute. 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-Password Attribute.

f 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 characters. The method is taken from the book "Network Security" by Kaufman, Perlman and Speciner \[9] 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. 

The String will contain c(1)+c(2)+...+c(i) where + denotes concatenation. On receipt, the process is reversed to yield the original password.

A summary of the User-Password Attribute format is shown below. The fields are transmitted from left to right.

Open

Type (IETF Value Number) - 2 for User-Password.

Length - At least 18 and no larger than 130.

String - The String field is between 16 and 128 octets long, inclusive.