iRule to decrypt and rewrite RADIUS User-Password AVP

Here is my final, working product. Any feedback, suggestions, or critiques are welcome.

when CLIENT_ACCEPTED {
set KEY "this-obviously-isn't-my-real-key:)"


set AUTHENTICATOR "[string range [UDP::payload 20] 4 20]"
set KEY_AUTH_MD5 [md5 "$KEY$AUTHENTICATOR"]

the below commands collapse the above 2 set commands commented out above
set KEY_AUTH_MD5 [md5 "$KEY[string range [UDP::payload 20] 4 20]"]







 START OF PACKET INTEGRITY VERIFICATION 
based off of https://devcentral.f5.com/codeshare/radius-server-using-apm-to-authenticate-users-1078 lines 39-64 and 78-86

EVALUATE REQUEST MESSAGE-AUTHENTICATOR
 RFC 2869 : A RADIUS Server receiving an Access-Request with a Message-Authenticator Attribute present MUST calculate the correct value
 of the Message-Authenticator and silently discard the packet if it does not match the value sent.

 Store only PAYLOAD in variable if Length field is valid (less than 4096 and less than payload length). prevent variable allocation if payload not valid.
 Octets outside the range of the Length field MUST be treated as padding and ignored on reception.
if {[binary scan [UDP::payload] cH2Sa16 QCODE IDENTIFIER QLEN Q_AUTHENTICATOR] != 4 || [set QLEN [expr {$QLEN & 0xFFFF}]] > [UDP::payload length] || $QLEN > 4096} {
    UDP::drop
    log local0. "Badly formatted RADIUS packet received"
    return
} else {
    set PAYLOAD [UDP::payload $QLEN]
}

we need to find AVP 80 within the UDP payload and set it to 32 bytes of 0s, but we are also checking that all other AVPs are formatted correctly
set ORIGINAL_MESSAGE_AUTHENTICATOR [RADIUS::avp 80 string]

for {set record_offset 20} {$record_offset < $QLEN } {incr record_offset $QAVP_LEN} {
 If an Attribute is received in an Access-Accept, Access-Reject or Access-Challenge packet with an invalid length, 
 the packet MUST either be treated as an Access-Reject or else silently discarded.
if {([binary scan $PAYLOAD @${record_offset}cc QAVP_TYPE QAVP_LEN] != 2) || ([set QAVP_LEN [expr {$QAVP_LEN & 0xFF}]] < 3) || ($record_offset+$QAVP_LEN > $QLEN) } {
    UDP::drop
    log local0. "Badly formatted AVP received"
    return
}
switch -- [set QAVP_TYPE [expr { $QAVP_TYPE & 0xFF}]] {
        80 {
            binary scan [string replace $PAYLOAD $record_offset [expr {$record_offset + 18}] [binary format ccH32 80 18 [string repeat 0 32]]] a* UNSIGNED_REQUEST
        }
    }
}


EVALUATE REQUEST MESSAGE-AUTHENTICATOR
 RFC 2869 : A RADIUS Server receiving an Access-Request with a Message-Authenticator Attribute present MUST calculate the correct value
 of the Message-Authenticator and silently discard the packet if it does not match the value sent.
if { "$ORIGINAL_MESSAGE_AUTHENTICATOR" != "" && ![CRYPTO::verify -alg hmac-md5 -key $KEY -signature $ORIGINAL_MESSAGE_AUTHENTICATOR $UNSIGNED_REQUEST]} {
    UDP::drop
    log local0. "Message-Authenticator (AVP 80) verification failed"
    return
}
else {
    we've verified the Message-Authenticator, no need to have the RADIUS server verify again
    RADIUS::avp delete 80
}
 END OF PACKET INTEGRITY VERIFICATION 




 For a minimum 8 character password and 44 character Yubikey OTP, we expect a minimum of 64 cypher bytes. 
set CYPHER_PW_OTP [RADIUS::avp 2 string]
if {[string length $CYPHER_PW_OTP]<64} {
    log local0. "password for [RADIUS::avp 1 string] too short BEFORE decryption"
    return
}





 START OF PASSWORD DECRYPTION 
based on suggestion from Stanislas Piron on https://devcentral.f5.com/questions/irule-to-decrypt-and-rewrite-radius-user-password-avp-62084answer159896
binary scan $KEY_AUTH_MD5 WW bx_64bits_1 bx_64bits_2
binary scan $CYPHER_PW_OTP W* USER_PASSWORD_W_LIST
set PASSWORD_LIST [list]
foreach {px_64bits_1 px_64bits_2} $USER_PASSWORD_W_LIST {
    lappend PASSWORD_LIST [expr { $px_64bits_1 ^ $bx_64bits_1 }] [expr { $px_64bits_2 ^ $bx_64bits_2 }]
    binary scan [md5 $KEY[binary format WW $px_64bits_1 $px_64bits_2]] WW bx_64bits_1 bx_64bits_2
}
binary scan [binary format W* $PASSWORD_LIST] A* PASSWORD_OTP
 END OF PASSWORD DECRYPTION 






 START OF OTP/PASSWORD SEPARATION 
 For a minimum 8 character password and 44 character Yubikey OTP, we expect a minimum of 52 cleartext bytes
if {[string length $PASSWORD_OTP] > 51} {
    set OTP "[string range "$PASSWORD_OTP" [expr {[string length $PASSWORD_OTP]-44}] end]"
    set PW "[string range "$PASSWORD_OTP" 0 [expr {[string length $PASSWORD_OTP]-45}]]"
    RADIUS::avp insert 17 "$OTP" string
    RADIUS::avp insert 17 "[string range "$PASSWORD_OTP" [expr {[string length $PASSWORD_OTP]-44}] end]"
} else {
    log local0. "password for [RADIUS::avp 1 string] too short AFTER decryption"        
    return
}
 END OF OTP/PASSWORD SEPARATION 





 START OF PASSWORD NULL PADDING 
this function has been collapsed into the next section

set PW_LENGTH [expr {[string length "$PW"]}]
set PW_MOD [expr {$PW_LENGTH%16}]
set PW_NULL_BYTES_TO_PAD [expr {(16-$PW_MOD)%16}]
set PW_PADDED "$PW[string repeat \x0 $PW_NULL_BYTES_TO_PAD]"

the below command collapses the above 4 set commands
set PW_PADDED "$PW[string repeat \x0 [expr {(16-[expr {[expr {[string length "$PW"]}]%16}])%16}]]"
 END OF PASSWORD NULL PADDING 





 START OF PASSWORD ENCRYPTION 
binary scan $KEY_AUTH_MD5 WW bx_64bits_1 bx_64bits_2
binary scan $PW_PADDED W* USER_PASSWORD_W_LIST
binary scan "$PW[string repeat \x0 [expr {(16-[expr {[expr {[string length "$PW"]}]%16}])%16}]]" W* USER_PASSWORD_W_LIST
binary scan [binary format a[expr {[string length $PW] + 16 - [string length $PW]%16}] $PW ] W* USER_PASSWORD_W_LIST
set PASSWORD_LIST [list]
foreach {px_64bits_1 px_64bits_2} $USER_PASSWORD_W_LIST {
    lappend PASSWORD_LIST [expr { $px_64bits_1 ^ $bx_64bits_1 }] [expr { $px_64bits_2 ^ $bx_64bits_2 }]
    binary scan [md5 $KEY[binary format WW $px_64bits_1 $px_64bits_2]] WW bx_64bits_1 bx_64bits_2
    binary scan [md5 $KEY[binary format W2 [lrange $PASSWORD_LIST end-1 end]]] WW bx_64bits_1 bx_64bits_2
}
binary scan [binary format W* $PASSWORD_LIST] A* CYPHER_PW
 END OF PASSWORD ENCRYPTION 




 Modify the RADIUS payload based on the above work
RADIUS::avp replace 2 "$CYPHER_PW" string
}