Digital Signing guidelines - NatWest

3. S/MIME Digital Signatures
S/MIME is the standard used to sign files that are sent to the customer. All payment files must also be signed using
S/MIME. Signing of all payment files must adhere to the S/MIME standard regardless of whether Smartcards or
HSMs are used to provide the signature. As S/MIME is a complex standard this section is included as a ‘primer’ for
customers who have no familiarity or experience with S/MIME.
It is not meant to cover all areas of the (S)MIME standard. It is meant only to give a high-level overview of how
S/MIME files are constructed, for use with Bankline Direct.
3.1 MIME
Bankline Direct requires that digitally signed files are submitted using the S/MIME format. S/MIME is an extension
of the MIME standard and is used to describe how to deliver signed and encrypted files.
Simply, MIME is formed of the following entities:
MIME Header – description of the MIME file contents
MIME Boundary – text delimiter to separate entities
MIME Part – a MIME file can have one or more MIME parts, each separated by a MIME boundary.
Each part will have headers describing the MIME part content
MIME Header
MIME Boundary
Describes the MIME file
Text delimiter
MIME Part N MIME Part 1
MIME Boundary
Text delimiter
MIME Part N+1 MIME Part 2
MIME Boundary
Text delimiter
MIME Part N+2 MIME Part 3
MIME Boundary
Text delimiter
For files with only one MIME Part, the boundary is not necessary:
MIME Header
Describes the MIME file
MIME Part MIME Part 1
3.2 S/MIME Digital Signatures
Digitally signed files can be submitted to Bankline Direct with detached or embedded (opaque) signatures.
Bankline only creates files with detached signatures.
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