coun·ter·sign 
(kountər-sīn′)
(kountər-sīn′)tr.v. coun·ter·signed, coun·ter·sign·ing, coun·ter·signs
To sign (a previously signed document) to vouch for the authenticity of the document.
n.
1. A second or confirming signature on a previously signed document. Also called countersignature.
2.
a. A sign or signal to be given to a sentry in order to obtain passage; a password.
b. A secret sign or signal given in answer to another.