Page 25 - ITU Journal - ICT Discoveries - Volume 1, No. 2, December 2018 - Second special issue on Data for Good

ITU JOURNAL: ICT Discoveries, Vol. 1(2), December 2018

hash component of type HashPointer contains a contentLocations ATTRIBUTE ::= {
value of the CMS message type DigestedData. WITH SYNTAX URIs
ID id-ContentLocations
}
This CMS type specifies the message digest
algorithm used to calculate the hash on the pointed URIs ::= SEQUENCE SIZE(1..MAX) OF uri URI
to object. Type DigestedData also indicates the
type of object being hashed is a SignedData block URI ::= UTF8String (SIZE(1..MAX))
header, a value of CMS type SignerInfo. In a A collection of items are signed by first calculating
precedingBlock attribute the type of object is a message digest over type ContentToBeSigned
indicated by an ASN.1 information object identifier defined as follows:
named id-signerInfo. Other types of objects can
also be identified in the DigestedData message. ContentToBeSigned ::= SEQUENCE
SIZE(1..MAX)OF content LocatedValue
In a precedingBlock attribute, the optional value of
type Pointers in type HashPointer may be omitted LocatedValue ::= OCTET STRING
when adjacent block locations are known. This Each value in type ContentToBeSigned is the data
may be the case when adjacent blocks are not located by one URI value in the contentLocations
distributed and reside in a common file system or attribute. Each URI contains a generalized form of a
database. More than one type of Pointer value may uniform resource locator (URL).
be used in series to fully qualify the location of a
distributed block. The defined set of pointer types 3.3 SignedData required attributes
is extensible to allow additional types of pointers
to be added as needed by an application.
CMS requires at least two attributes to be present
if any signed attributes are included in a
3.2 Detached content
SignedData message. These attributes are defined
in CMS and named contentInfo and messageDigest.
The data content of a SignedData block is optional
to include and need not be present in a given When SignedData is used as a blockchain block, the
contentInfo attribute value is set to indicate the
message. In a secure electronic mail (email) type of content in the block is ordinary data, rather
application that provides signed email messages than one of the other cryptographic message types
using the SignedData type, the detached message defined in CMS.
content and the signature are located in separate
parts of the same email message. This allows the The required messageDigest attribute contains the
content to be displayed for the email recipient hash of the content. The content is bound to the
even when an email agent cannot process signed other signed attributes cryptographically under
email, or when verification of the digital signature the digital signature of the signer. There may be
on the detached content fails.
any number of additional attributes included in
this binding at the signers choice, and these may be
In other application contexts, such as the cloud and of any type or format.
Internet of things (IoT) environments, the location
of detached SignedData content must be identified 3.4 Blockchain required attributes
and made available for verification of the signature
on the message content. When the location of In a SignedData blockchain, both the timeStamp
detached data content is not known to blockchain attribute and precedingBlock attributes shown in
participants, a SignedData message signer can Fig. 1 must be included in the SignerInfo block
include a content location attribute in the signed header. The value of a timeStamp attribute in a
attributes of the block header. This attribute can be given block header may contain a choice of either a
used to locate detached data content distributed in locally sourced synchronized time, or a trusted
one or more locations. A content location attribute timestamp token. A trusted timestamp is based on
can be defined as a series of one or more uniform a coordinated time source, such as one of those
resource identifier (URI) [6] values in ASN.1 as specified in the X9.95 standard [7]. The type of
follows: time value used may vary by block, but should
meet the requirements agreed to by the blockchain
participants. For blocks associated with tagged,

© International Telecommunication Union, 2018 3