295. The principle of legality requires that an individual cannot be found guilty of any criminal offence that “did not constitute a criminal offence, under national or international law, at the time when it was committed”.653 In cases where there has been a change in available sentencing powers, this prohibition of the retroactive application of criminal law forbids the imposition of a penalty in excess of the maximum permissible sentence at the time the offence was committed. The imposition of a lighter sentence will not violate the prohibition of retroactive offences.654 This prohibition is non-derogable.655 296. The principle of legality is not violated when the offence was already punishable under international law, such as international humanitarian law, at the time the conduct occurred even if it was not punishable under domestic law.656 For instance, war crimes, crimes against humanity and acts of torture are punishable under international law. Therefore, individuals may be prosecuted for these offences even where domestic law does not stipulate that these are criminal offences at the time the conduct occurred.657 297. The principle of legality also creates a duty to define all criminal offences and corresponding punishment clearly and precisely by law to avoid arbitrary application of the law.658 This requirement means that, at any given time, an individual must be able to know, or be able to find out, what the law is and be able to regulate his or her conduct accordingly. 659 This requirement is not met if the individual could not reasonably have been aware that the act or omission was a criminal offence at the time of the conduct.660 The obligation to define crimes in precise unambiguous language implies that definitions of crimes must be strictly construed and that any ambiguity must be determined in favour of the accused.661 If any necessary element of a criminal offence cannot be proven to have existed at the time the conduct occurred, then a conviction for the act or omission violates the principle of legality.662 653 ICCPR, Art. 15; ACHR, Art. 9; UDHR, Art. 11(2); OSCE Copenhagen Document, op. cit., note 80, para. 5.18. ICCPR, Art. 15; ACHR, Art. 9. 655 “No derogation from articles (…) 15 (…) may be made”. ICCPR, Art. 4(2); ACHR, Art. 27(2); UN HRC, General Comment No. 29, op. cit., note 111, para. 7. 656 ICCPR, Art. 15(2): “[n]othing in this article shall prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognized by the community of nations”. 657 International law refers to both international treaty law and customary international law. Nowak, CCPR Commentary, op. cit., note 93, p. 360. 658 ICCPR, Art. 15; UN HRC, General Comment No. 29, op. cit., note 111, para. 7; Inter-American Court of Human Rights, Castillo Petruzzi et al v. Peru, 30 May 1999, para. 121, <http://www.corteidh.or.cr/docs/casos/articulos/seriec_52_ing.pdf>; Legal Digest of International Fair Trial Rights (Warsaw: OSCE/ODIHR, 2012), pp. 186-189, <http://www.osce.org/odihr/94214>. 659 ICTY, Prosecutor v. Stanislav Galić, Case No. IT-98-29-T, 5 December 2003, para. 93, <http://www.icty.org/x/cases/galic/tjug/en/gal-tj031205e.pdf>; Legal Digest of International Fair Trial Rights, ibid., p. 188. 660 ICTY, Prosecutor v. Mitar Vasiljevic, Case No. IT-98-32-T, 29 November 2002, para. 193, <http://www.icty.org/x/cases/vasiljevic/tjug/en/vas021129.pdf>. 661 UN HRC, General Comment No. 29, op. cit., note 111, para. 7; ICTY, Prosecutor v. Galić, op. cit., note 659, para. 93: “[t]he effect of strict construction of the provisions of a criminal statute is that where an equivocal word or ambiguous sentence leaves a reasonable doubt of its meaning which the canons of construction fail to solve, the benefit of the doubt should be given to the subject and against the legislature which has failed to explain itself”; Inter-American Court of Human Rights, Petruzzi et al v. Peru, op. cit., note 658, para. 121. 662 UN HRC, David Michael Nicholas v. Australia, Communication No. 1080/2002, 19 March 2004, para. 7.5, <http://tbinternet.ohchr.org/_layouts/treatybodyexternal/Download.aspx?symbolno=CCPR%2FC%2F80%2FD%2F1080%2F200 2&Lang=en>. 654 129