util/encodings.go
func AllAlternateNameWithTagAreIA5(ext *pkix.Extension, tag int) (bool, error) {
	var seq asn1.RawValue
	var err error
	// Unmarshal the extension as a sequence
	if _, err = asn1.Unmarshal(ext.Value, &seq); err != nil {
		return false, err
	}
	// Ensure the sequence matches what we expect for SAN/IAN
	if !seq.IsCompound || seq.Tag != asn1.TagSequence || seq.Class != asn1.ClassUniversal {
		err = asn1.StructuralError{Msg: "bad alternate name sequence"}
		return false, err
	}

	// Iterate over the sequence and look for items tagged with tag
	rest := seq.Bytes
	for len(rest) > 0 {
		var v asn1.RawValue
		rest, err = asn1.Unmarshal(rest, &v)
		if err != nil {
			return false, err
		}
		if v.Tag == tag {
			if !IsIA5String(v.Bytes) {
				return false, nil
			}
		}
	}

	return true, nil
}
func AllAlternateNameWithTagAreIA5(ext *pkix.Extension, tag int) (bool, error) {
	var seq asn1.RawValue
	var err error
	// Unmarshal the extension as a sequence
	if _, err = asn1.Unmarshal(ext.Value, &seq); err != nil {
		return false, err
	}
	// Ensure the sequence matches what we expect for SAN/IAN
	if !seq.IsCompound || seq.Tag != asn1.TagSequence || seq.Class != asn1.ClassUniversal {
		err = asn1.StructuralError{Msg: "bad alternate name sequence"}
		return false, err
	}

	// Iterate over the sequence and look for items tagged with tag
	rest := seq.Bytes
	for len(rest) > 0 {
		var v asn1.RawValue
		rest, err = asn1.Unmarshal(rest, &v)
		if err != nil {
			return false, err
		}
		if v.Tag == tag {
			if !IsIA5String(v.Bytes) {
				return false, nil
			}
		}
	}

	return true, nil
}
// AllAlternateNameWithTagAreIA5 returns true if all sequence members with the
// given tag are encoded as IA5 strings, and false otherwise. If it encounters
// errors parsing asn1, err will be non-nil.