authenticate (v.)

• accept
• accredit
• affirm
• approve
• attest
• authorize
• autograph
• avouch
• back
• bolster
• buttress
• certify
• circumstantiate
• confirm
• corroborate
• cosign
• countersign
• demonstrate
• document
• endorse
• fortify
• initial
• justify
• notarize
• pass
• permit
• probate
• prove
• ratify
• reinforce
• sanction
• seal
• second
• sign
• strengthen
• substantiate
• support
• sustain
• test
• try
• undergird
• undersign
• underwrite
• uphold
• validate
• verify
• visa
• vise
• warrant